1. Field of the Invention
The present invention relates to an optical wavelength filter for taking out the light of a particular wavelength from input light, more particularly to an optical wavelength filter utilizing the acoustooptic effect.
2. Description of the Related Art
When an acoustic wave and light are propagated through a solid, as the propagation of the acoustic wave proceeds, a refractive index of an optical waveguide through which the light is propagated changes periodically, and the light is polarization transformed due to a periodical change of the refractive index. Since wavelength dispersion takes place during polarization transform, the light of a wavelength which satisfies the phase matching condition is strongly transformed by polarization transform. The optical wavelength filter which uses this acoustooptic effect has a characteristic such that it is capable of high-speed motion resulting in a wide variable range of tuning and hence it can select a variety of channels. The above optical wavelength filter also has a characteristic that it can concurrently select the light of a plurality of wavelengths.
As an example of the optical wavelength filter which uses the conventional acoustooptic effect, an optical wavelength filter of a structure shown in FIG. 1 is proposed.
The conventional optical wavelength filter shown in FIG. 1 is described in the technical digest "Low-sidelobe integrated acoustooptic tunable filter surface acoustic waves",Topical Meeting on Integrated Photonics Research, Apr.13-15, 1992.
In FIG. 1, near the surface of substrate 81 made of a lithium niobate (LiNbO.sub.3) crystal of Y axis cut X axis propagation, titanium-doped optical waveguide 82 is formed. On substrate 81, first acoustic wave absorber 85.sub.1 for absorbing an elastic surface wave is mounted at the input end side of optical waveguide 82, and elastic surface wave excitation electrode 83 for generating the elastic surface wave is formed at the output side of first acoustic wave absorber 85.sub.1. Further, a second acoustic wave absorber 85.sub.2 for absorbing the acoustic wave which is the elastic surface wave is mounted at the output end side of optical waveguide 82.
With a structure like this, interaction area 84 in which the acoustic wave acts on the light is interposed between first acoustic wave absorber 85.sub.1 and second acoustic wave absorber 85.sub.2, and in which a periodical refractive index change of the light is produced due to periodical grating caused by the elastic surface wave excited by elastic surface wave excitation electrode 83.
Now, if first linear polarization (hereinafter referred to as TE polarization) composed of the light of a plurality of wavelength (.lambda..sub.0 to .lambda..sub.n) having an electric field component horizontal to substrate 81 is inputted from the input end, the light of a particular wavelength (.lambda..sub.1) satisfying the phase matching condition having an electric field component perpendicular to substrate 81 is transformed to a second linear polarization (hereinafter called as TM polarization) under the influence of a periodical refractive index change of interaction area 84.
By passing the light outputted from the optical waveguide 82 through a polarization separation element, not shown, only the light of wavelength (.lambda..sub.1) transformed to TM polarization is outputted from the polarization separation element. According to this effect, it serves as an optical wavelength filter which passes light of a particular wavelength.
A weighting method applied for controlling the acoustooptic effect has been known as a method for decreasing a side-lobe level of the output of the optical wavelength filter.
This is a method for controlling a high degree mode which is generated when the acoustic wave acts on the light, by changing a width or a space of each finger electrode portion which constitutes elastic surface wave excitation electrode 83 composed of a reed screen type electrode and arranging the finger electrode portions so that the energy distribution of the elastic surface wave agrees with a predetermined function (for example, a Hamming's function) with reference to the position thereof.
With the optical wavelength filter shown in FIG. 1, the weighting method is applied in such a way that elastic surface wave excitation electrode 83 is formed with a reed screen type electrode of a circular arc shape and by making the energy distribution of the elastic surface wave agree with a predetermined function by converging the elastic surface wave within interaction area 84.
In the conventional optical wavelength filter described above, although the elastic surface wave is converged by making the elastic surface wave excitation electrode into a simple circular arc form, this type of structure is limited to a case that a crystal, in which the direction of a normal line of a reed screen type electrode and the direction of propagation of the real elastic surface wave coincide with each other (there is no deflection in the propagation direction), for example, such as lithium niobate of Y axis cut X axis propagation is used as a substrate.
In case a crystal having deflection in the propagation direction of the elastic surface wave, for example, lithium niobate of X axis cut Y axis propagation having a larger electro-mechanical coupling coefficient is used as a substrate, it is impossible to converge the elastic surface waves at a desired position, and hence it has been unable to apply this structure to actual use.